The invention relates to a process and switching means for regulating heating output of a heating device, especially a heating device intended for use as an auxiliary heater in vehicles, or as a stationary heater, where the heat output is produced via a heat exchange medium. More particularly, wherein heat output is controlled in dependence upon a control temperature and predetermined temperature threshold values. Suitable heat exchange media are a liquid, e.g., water, or a gas, e.g., air.
German Offenlegungsschrift No. 30 31 410 (U.S. Pat. No. 4,411,385) shows a control device for operating a heating apparatus, particularly for use in vehicles, in which, for the purpose of heat output control, a control temperature, e.g., the temperature of a liquid heat exchange medium, is determined by a sensor, and the heating device is operated with a variety of possible heating stages, dependent upon empirically determinable temperature threshold values. Upon triggering of a first temperature threshold value, for instance, a vehicle blower is activated, and after passing a second, higher, temperature threshold value, operation of the heater is switched to a partial load range and then subsequently back to full load range operation, if appropriate.
German Pat. No. 30 24 983 (U.S. Pat. No. 4,436,064) didcloses a process and control means for determining a preheating energy quantity, in which, for the preheating of a heatable object or space prior to an intended time of usage, and particularly for the preheating of a motor vehicle, a selected preheating result can be automatically obtained with the lowest possible energy consumption and utmost amount of safety. Here too, several variable heating stages can be passed for preheating, dependent upon the tempeature differences between the initial temperature and the desired heat-up temperature.
In all of these previously known heating control devices, there is short-term deactivation and activation of the heating device within brief control intervals during the vehicles's operation, for preheating as well as for main heating, particularly, if the heat output requirement is small. This not only reduces the service life of the burner units, but causes the entire heating device to have a tendency towards coking and sooting. Such unfavorable operating conditions may result in reduced operational safety, as well as diminished ease in starting the device. Under such circumstances, the heating device operates with low efficiency, relative to the heat energy produced and electrical energy consumed, and consequently presents the drawback of relatively high energy consumption. This is an especially negative factor, when the electrical energy supply for the heating device is derived from a battery, as in a motor vehicle.
It is, therefore, a primary object of the invention to provide a process and a switching apparatus for regulating the heat output of a heating device operating with various levels of heating performance and with a heat exchange medium, which results in a more efficient operation of the heating device, while eliminating coking or sooting of the heating device, and also overcoming the other drawbacks and problems discussed above.
In accordance with a preferred embodiment of the invention, a process for controlling the heat output of a heating device employing a heat exchange medium involves determining a temperature gradient for heating the heat exchange medium between two temperature threshold values and then controlling the operational level of the heating device in dependence upon the slope of the temperature gradient. A corresponding switching arrangement for performing such process utilizes a sensing device to measure the temperature gradient and is connected with a control unit which activates a heat output control dependent upon the slope of the temperature gradient.
In the process of controlling the heat output, in accordance with the preferred embodiment of the invention, the temperature gradient, for the heating of the heat exhanger medium (gas or liquid) between two temperature threshold values (these values being empirically determinable and prespecified for controlling heat output), is ascertained by the application of a control temperature value. The slope of this temperature gradient is a measure of the actual heat output requirement, or is a measure of the ratio of heat output produced to energy consumed by the heating device.
A determination of the temperature gradient and its slope permits an increase and an extension of the intervals of between control pauses and the reactivation of the heating device, eliminating undesirable control fluctuations with resultant changes in operating conditions of the device. The electrical energy consumption for operating the heating device can be lowered, and thus an efficient heating performance encompassing various preheating stages, as well as main heating is ensured. As there is no necessity of repeated reactivation and ignition of the heating device within short intervals, the service life of the burner units is extended, and the problems of coking as well as soot formation are eliminated in the heating device.
The characteristic measured to determine the slope of the temperature gradient is the time required for heating the heat exchange medium from the first temperature threshold value to the second temperature threshold value. Such a heating period can easily be determined using the principles of control technology and can be processed in an uncomplicated manner with regard to influencing the heat control process.
The measured heating period is coordinated with the respective characteristic operating ranges of the heating device in the following manners. A first predetermined time value is selected for the time interval required to heat the heat exchange medium from the first to the second threshold values. If the measured heat-up time falls below this preset time value during the heat-up interval, the heating device is switched off because, presumably, there is an operational malfunction, or an error in operating the heating device. In this instance, the temperature gradient is very steep, which means that the heat exchange medium is heated from the first tempeature threshold value to the second temperature threshold value within a short time, resulting in a very low heating output by the heating device. In such case, the heating device is automatically switched off, in order to avoid damage thereto or to prevent a triggering of the overheating safeguard device, and/or to eliminate a heating operation which is not oriented towards actual heating demand. When a liquid is used as a heat exchange medium it may continue to circulate in the device by way of a circulating pump, for the purpose of dissipating heat, or alternatively, the circulating pump itself may also be switched off.
A second empirically predetermiend, preset time value for the heating period is used which is greater than the first predetermined time value. If the measured heat up time value falls below this second preset value, the temperature threshold value for changing the operating condition of the heating device is lowered. Such a procedure applies when the heating device supplies a higher output than is necessary for heating, and occurs particularly, when heating during the warmer transitional seasons, with reduced control valves or closed vent flaps, or when heating with the internal combustion engine warmed up. Lowering of the temperature threshold value for a change in operating conditions of the heating device avoids having the heating device successively switched on and switched off within a short time interval, when such is not necessary on the basis of the required heat demand.
Furthermore, in the process, the heat output of the device is controlled in the usual manner depending upon the control temperature in variously possible heating stages, when the second preset value for the heating period is exceeded. Accordingly, this process enables the heating device to work with the normal basic control operation for the supply of heat energy.
The temperature range used for determining the temperature gradient is selected to fall between the lower temperature threshold value that triggers the activation of the vehicle blower, and the higher temperature threshold value which effects a reversal of the heating device to partial load conditions, both of these value having been previously present. Empirically determinable temperature threshold values range from between approximately 50.degree. C. for the lower threshold value and 80.degree. C. for the higher threshold value.
The switching arrangement, in accordance with the invention, facilitates easy implementation of the inventive control process. If appropriate the switching arrangement can be realized by a microprocessor component or a highly integrated logic module.
In the preferred embodiment switching arrangement, there is cooperation between a sensor device for determining the temperature gradient, a control unit connected with the sensor device, and the heat output regulating control of the heating device, for the purpose of activating the heat output control means of the heating device, dependent upon the temperature gradient determined by the sensor device, such that the heat supplied by the heating device is based on demand, while simultaneously avoiding critical operating conditions.
The sensor device, appropriately, comprises a timer for measuring the heat-up time between threshold values to determine the temperature gradient slope. The heat-up time is the governing regulating value here, and based thereon, with consideration to special operating ranges of the device, there is corresponding activation of the heating output control via a control unit of the switching arrangement.
Additionally, a malfunction indicator device can be provided which is activated by the control unit and which issues a malfunction signal when, based on readings sensed by the sensor device, the heat exchange medium heats up very rapidly. The cause for such rapid heating may be an operational malfunction or an error on the part of the operator of the heating device. In a given case, the circulating pump used for circulating the liquid heating exchange medium can continue operation to dissipate residual heat, or, alternatively, it may also be turned off.
By way of the switching arrangement, the heat output regulating control for operation of the heat output control means is changed such that, in comparison with the basic control operation, longer control pauses between changes in switch conditions of the heat output control means are attained. This permits a reduction of electrical energy consumption of the heating device and leads to an increase in total performance reliability.
Appropriately, comparator controls are used, for determining individual switch conditions of the control unit, in the switching arrangement.
Upon reaching the first, lower temperature threshold value, the timer can be switched on to measure time, and it can be switched off when the second temperature threshold value is reached. The time difference can be ascertained by means of a differential determining device, in a given case.
It is to be understood that the switching data may also be integrated into a microprocessor or other highly integrated logic component.